The following meanings for the abbreviations used in this specification apply:
3GPP 3rd generation partnership project
BSC Base station controller
eNode-B LTE base station (also referred to as eNB)
E-UTRAN Evolved universal terrestrial radio access network
GSM Global system for mobile communications
LTE Long term evolution
NRT Non-real time
RAN Radio access network
RAT Radio access technology
RIM RAN information management
RLF Radio link failure
RNC Radio network controller
RRC Radio resource control
RSRP Reference signal received power
UE User equipment
UMTS Universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
Embodiments of the present invention relate to mobile radio communications with focus on inter radio access technique (inter-RAT) operation, for instance, coverage limited handover between overlaying radio access networks. Embodiments of the present invention aim on inter-RAT handover improvement.
Namely, in near future, LTE will be deployed as second or third radio access network (RAN) of an operator. In most areas several radio technologies will exist as overlay networks. In the beginning, LTE system will only be deployed at some hotspot area embedded in wide-ranging legacy 2G/3G overlay coverage. An example for this is illustrated in FIG. 8. Many inter-RAT handovers will happen either due to the limited LTE coverage or triggered by dedicated operator policies in terms of service or load based traffic steering.
The problem underlying the present application relates to a missing appropriate procedure for connection re-establishment in case of RLF detection in a multi-RAT environment.
3GPP standard already foresees a specific recovery strategy for UE that detects a radio link failure (RLF). After detecting RLF, the UE stays in RRC connected mode and tries to recover by sending a RRC CONNECTION RE-ESTABLISHMENT REQUEST message to the next best serving eNB staying on same carrier frequency, i.e. UE is working in intra-RAT/intra-frequency mode. The re-establishment method allows a rather fast recovery of the connection without lengthy authentication procedures and security activation provided that the concerned cell is prepared, i.e. it has a valid UE context. If there is no UE context prepared at the targeted eNB, the UE changes to status RRC_IDLE.
FIG. 9 shows the intra-LTE recovery phases as specified in 3GPP TS 36.300. As shown, in a first phase, initiated by radio link quality problem detection (signal quality of the serving cell falls below a certain threshold) operation is continued and it is checked if the signal quality recovers within a time frame T1. If the UE does not recover in the same cell, i.e. timer T1 expires, it is determined that a radio link failure (RLF) is present and the second phase, (timed by timer T2), is started where the UE, still in status RRC-connected, is selecting a different cell from the same eNB and tries to resume activity by sending the RRC Connection re-establishment. If this is also not successful, the UE goes back to idle (RRC_IDLE) and has to pass to the normal re-connection procedure.
The problem is that in case of multi-RAT layout an appropriate procedure for connection re-establishment to another RAT in case of RLF detection is missing.